Prevalence and severity of disease of the protozoan parasite Toxoplasma gondii varies geographically in the wide array of hosts T. gondii infects. These facts highlight the need to discover the transmission dynamics and the genetic relationship among strains that are causing the wide spectrum of disease states identified in nature. Specifically, our research addresses the emergence and re-emergence of parasitic zoonotic diseases that have complex life-cycles with multiple routes of transmission and impact people and animals that co-exist in the same ecological niche. We seek discoveries in these areas to support the development of new diagnostic tools, discover fundamental paradigms governing virulence shifts in parasitic protozoa and ultimately develop efficacious anti-protozoal strategies that mitigate the spread of disease. In ongoing studies we have so far isolated >80 strains from an outbreak of protozoal meningoencephalitis in marine mammals off the coast of Southern California. Collectively, these isolates define a new "wild" clade of T. gondii organisms that are quite different from the Type I, II and III genotypes that commonly infect people worldwide. We now refer to the new clade of strains as Type X. Ten distinct strains have so far been identified that all appear to be related by reassortment of a limited set of divergent alleles across the loci examined. Hence, the most parsimoniuos explanation for the origin of these strains is that they appear to be the result of a sexual cross. Reassortment in viruses is a common paradigm for the evolution of new pathogenic "Types", whether this is common to parasites remains to be established and discovering whether parasites utilize sexual recombination to evolve to occupy new niches or cause new disease will be an instrumental finding.